The invention relates to an infrared radiation source (radiator) having a radiation-transparent, gas-tight tube of electrically insulating material in which is arranged a thermal conductor made of carbon strip, having two ends which are respectively electrically and mechanically connected to bushings made of molybdenum and/or tungsten and/or tantalum, wherein the bushings are respectively connected via current feed-through leads with electrical connections projecting from the tube.
Such infrared radiation sources are known from German published patent application DE 100 29 437 A1. In particular, FIGS. 14 and 15 therein show the contacting between a carbon strip and a bushing made of molybdenum, the carbon strip having a coiled form and the bushing surrounding this carbon coil internally and externally. Graphite paper is arranged between the bushing and the carbon coil. A noble metal paste, or a metallic coating, for example made of nickel or a noble metal, applied to the ends of the carbon strip, can be arranged between the graphite paper and the carbon strip. The metallic coating on the ends of the carbon strip prevents a degeneration of the carbon material at the transition between carbon strip and graphite. However, at high temperatures in operation of the carbon strip, a diffusion of carbon from the graphite paper into the molybdenum of the bushing takes place with this kind of contacting. This leads to the formation of molybdenum carbide and consequently, at operating temperatures of the carbon strip substantially above 1300° C., to embrittlement or a break-up of the bushing, since molybdenum carbide has a higher electrical resistance, a different specific volume and also a markedly lower strength in comparison with molybdenum. The whole contact disintegrates after a few hours or weeks, depending on how high the carbon strip temperature is.
European published patent application EP 0 881 858 A2 discloses an infrared radiation source with a heat conductor made of carbon fibers. The ends of the heat conductor are electrically contacted by metallic connections, wherein contact strips made of molybdenum, among others, are used in direct contact with the heat conductor. The heat conductor can have at its ends a metallic coating, for example of gold. Furthermore, graphite paper can be arranged between the contact strips and the heat conductor, which is in direct contact with the contact strips and thus again leads to a diffusion of carbon from the graphite paper into the molybdenum and consequently to the formation of molybdenum carbide, with the problems already described above. With the connections illustrated between the contact strips and the heat conductor, it can be seen that respectively only a point or strip form pressing of the contact strip onto the heat conductor takes place. In operation of the infrared radiation source, a very high current flux exists in these regions, producing temperatures of above 1100° C. there, and accelerating the diffusion of carbon into the molybdenum. The impoverishment of the heat conductor in carbon leads to a point increase of resistance and a further temperature increase, having as a consequence the burn-up of the heat conductor.
German Patent DE 199 17 270 C2 discloses an infrared radiation source with an elongate carbon strip, which is contacted on both sides by contact pads.
European Patent EP 0 464 916 B1 discloses a high temperature-resistant composite member consisting of at least one part made of molybdenum or a molybdenum alloy and at least one part made of graphite, connected together with a zirconium solder. Between the part made of molybdenum or molybdenum alloy and the zirconium solder, a two-layer intermediate layer is arranged, which is formed of a layer of vanadium and a layer of one of the elements tungsten, tantalum, or niobium.